1. Field of the Invention
The present invention generally relates to Flexible Layer One (FLO) for the Global System for Mobile Communications (GSM)/Enhanced Data Rates for Global Evolution (EDGE) Radio Access Network (GERAN) and, more particularly to, a system and method for decoding signalling messages on FLO half-rate channels.
2. Detailed Description of the Related Art
Presently, a new layer, i.e., Flexible Layer One (FLO) for the GERAN is being standardized in Release 6 of the Third Generation Partnership Project (3GPP). This standardization is being performed based on the same principles that are used in a Universal Terrestrial Radio Access Network (UTRAN). Instead of utilizing fixed coding schemes, FLO provides a framework that permits a particular coding scheme to be defined and optimized at call setup according to the quality of service (QoS) requirements of the service to be supported, such as IP Multimedia Subsystem (IMS) services.
The main advantage of FLO is that the configuration of the physical layer (for example channel coding and interleaving) is specified at the call setup. As a result, the support of new services can be handled smoothly without the need to specify new coding schemes in each released version of the new services.
Through several enhancements, such as reduced granularity and flexible interleaving, the radio bearers offered by FLO would not only fulfill the IMS requirement for flexibility and performance, but also greatly improve link level performance of real-time IMS services in comparison to the link performance that is achieved in GERAN Release 5. Granularity is the extent to which a system contains separate components, for example, granules. The more components in a system, i.e., the greater the granularity, the more flexible it is.
Control plane messages and Radio Link Control/Medium Access Control (RLC/MAC) messages are signalling messages that are each associated with a specific service. These signalling messages need to be carried with the specific service. Even in adverse channel conditions, the reliable transport of such signalling messages must be achieved throughout a radio access network. As a result, a high level of signalling protection is implemented in GSM communications. For example, throughout every network, as established in 3GPP TS 45.003 V6.5.0 “Radio Access network; Channel Coding,” Coding Scheme 1 (CS-1) is always used to guarantee consistent performance in every cell.
The flexibility offered by FLO provides the ability to change the level of signalling protection. It is also possible to multiplex signalling messages with other radio bearers. While this kind of flexibility is desirable in the user plane, it is generally avoided in the control plane because it can lead to inconsistent signalling performance in the network (for example, handover commands become unavailable in certain areas). As established in 3GPP TR 45.902 V6.6.0, “Radio Access network; Flexible Layer One,” the configuration of the signalling is fixed in order to guarantee consistent signalling message performance.
A full rate channel relates to the instance where a mobile station (MS) is accessed every 3rd time slot, for example, time slots 1 and 3, time slots 2 and 4, or time slots 3 and 6. With half rate channels, the MS is accessed every 6th time slot. For full rate and half rate channels, the first transport format combination (TFC) of the transport format combination set (TFCS) is used to send signalling messages. Within this first TFC, only the transport channel carrying the transport blocks of the signalling message is active with a fixed transport format (for example, 184 bits of transport block data and 18 bits of cyclic redundancy check (CRC) data).
In the case of full rate channels, the Medium Access Control (MAC) layer sends the signalling transport block once. However, for half rate channels the MAC layer transmits the same signalling transport block twice in a row. Here, coded bits of the same transport block become located in two consecutive radio packets. As a result, the effect is the same as if the interleaving depth was twice the interleaving used for one radio packet. That is, both radio packets possess the same transport format combination indicator (TFCI) at Layer One.
However, the TFCI is the first item that is read by a decoder. As a result, unless double decoding is permitted, the radio packet will be lost if the TFCI is misread. Therefore, there is a need for a decoding procedure for permitting the correct processing of misread TFCIs on half rate channels and for minimizing the effects of misread TFCIs, especially in the transport blocks of signalling messages.